Battery damage clarification

[prState]

One thing I'm not clear on, is when batteries are discharged too low, does battery damage occur in degrees, or is it all or nothing? That is, can it be classed as minor, moderate, severe...? And does it differ with different types of batteries?

Thanks.

 

[flecc]

There are many variations in this, not just in the types but in differing results and also depending on age and quality.

For example, NiMh cells shouldn't be discharged below 1 volt per cell minimum, or they might fail to accept a recharge and then be useless. That can happen with discharge down to about 0.9 volts, but at another time a cell of the same make and type can be recharged from only 0.4 volts.

I've no knowledge of what Li-ion cells do in this circumstance, since they haven't been around that long and are cosseted within protection circuitry that cuts off supply well before the damage point. Since they don't have a storage discharge loss and retain whatever they have, there's no accidental occurrence either.

The general rule with all types is to recharge at least every three months, and the more they go beyond that without a charge, the less likely that they will still be usable.
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[rooel]

Although NiMh do not have the protection circuitry found in Lithium packs, I wonder if some motor/controller systems supply that themselves.

I have found with the Sram Sparc system that the motors no longer draw power from either NiMh or Lithium when these batteries fall to around 15 volts. The Lithium obviously cuts the power supply when its protection circuit senses a fall to 15 volts, but the NiMh without such a protection circuit seems to provide the same result at the same voltage.

 

[flecc]

Yes they do Rooel, all I've known have a floor voltage cut-out which is comfortably above the NiMh damage point.

The problem with NiMh is the natural loss of 1 to 1.5 % of charge each day, meaning that if left uncharged too long, they can get beyond redemption.

Conversely, leaving them on a trickle charge can damage them if the trickle rate is too high, and the really safe level for that can still mean some loss over time.
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[Ian]

One of the main problems with a number of cells in series, as in all bike batteries, is that all the cells will not reach exhaustion at exactly the same time, typically some cells will fall bellow a "safe" voltage before the pack as a whole reaches its nominal minimum. This is not normally a problem unless the pack is discharged below its minimum, in which case the weaker cells will discharge to a point where current from the stronger cells is forced through them in reverse, this is extremely damaging making the weak cells even weaker so they will reach exhaustion and reversal earlier during the next discharge cycle, and so on leading to rapid failure.